trans not shifting

99 GTP just had a new cat welded in then the next day the trans would not go in to drive ..it takes reverse good but it will only take 1st gear and it slips real bad...

Did you unhook the battery before welding on the car? Did you take the car for a drive after the new cat?

If not, what may have happened is welding on the car has the possibility of shorting ECU's.

it ran fine for a day .the next morn it wouldnt take gear..i drove it all day after they replaced the cat ....